merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/RelNotes/1.5.5.2.txt

@@ -0,0 +1,27 @@
+GIT v1.5.5.2 Release Notes
+==========================
+
+Fixes since v1.5.5.1
+--------------------
+
+ * "git repack -n" was mistakenly made no-op earlier.
+
+ * "git imap-send" wanted to always have imap.host even when use of
+   imap.tunnel made it unnecessary.
+
+ * reflog syntax that uses time e.g. "HEAD@{10 seconds ago}:path" did not
+   stop parsing at the closing "}".
+
+ * "git rev-parse --symbolic-full-name ^master^2" printed solitary "^",
+   but it should print nothing.
+
+ * "git commit" did not detect when it failed to write tree objects.
+
+ * "git fetch" sometimes transferred too many objects unnecessarily.
+
+ * a path specification "a/b" in .gitattributes file should not match
+   "sub/a/b".
+
+ * various gitweb fixes.
+
+Also comes with various documentation updates.